feat(adapters): ShardAdapter contract + read-only FolderAdapter (WP-0007 T3)

Versioned ShardAdapter ABC (shard_id/profile/keys/read mandatory; optional verbs
raise NotSupported by default = honest absence). FolderAdapter reads a dir of
Markdown into Pages (relpath=key, mtime=source_rev, provenance envelope),
declaring a validated read-only file-store profile. 4 tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_folder_adapter.py

@@ -0,0 +1,44 @@
+"""Tests for the FolderAdapter (SHARD-WP-0007 T3)."""
+
+import pytest
+
+from shard_wiki.adapters import FolderAdapter
+from shard_wiki.model import Identity, NotSupported, Verb
+
+
+def _make_shard(tmp_path, files: dict[str, str]) -> FolderAdapter:
+    for rel, text in files.items():
+        p = tmp_path / rel
+        p.parent.mkdir(parents=True, exist_ok=True)
+        p.write_text(text, encoding="utf-8")
+    return FolderAdapter("shardA", tmp_path)
+
+
+def test_keys_and_read(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "# Home", "sub/Topic.md": "topic body"})
+    assert set(shard.keys()) == {"Home", "sub/Topic"}
+    page = shard.read("sub/Topic")
+    assert page.identity == Identity("shardA", "sub/Topic")
+    assert page.body == "topic body"
+    assert page.envelope.source_shard == "shardA"
+    assert page.envelope.source_rev is not None  # mtime stamp
+    assert page.placements[0].path == "sub/Topic.md"
+
+
+def test_read_missing_raises_keyerror(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "x"})
+    with pytest.raises(KeyError):
+        shard.read("Nope")
+
+
+def test_profile_is_valid_and_read_only(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "x"})
+    prof = shard.profile()  # .validate() already called inside
+    assert prof.supports(Verb.READ)
+    assert not prof.supports(Verb.WRITE)
+
+
+def test_unsupported_write_is_honest(tmp_path):
+    shard = _make_shard(tmp_path, {"Home.md": "x"})
+    with pytest.raises(NotSupported):
+        shard.write("Home", "new")